The relevant active implanted medical devices typically comprise a generator containing the various electronic circuits and a power supply for the device. The generator is connected electrically and mechanically to a lead equipped with electrodes for the stimulation of tissue. Of particular interest are those devices that have leads equipped with electrodes for intracardiac stimulation, making it possible to detect the potentials of depolarization of the myocardium, and to deliver as needed the stimulation pulses produced by the generator.
An important parameter of an implanted lead is its impedance, because this parameter conditions the current necessary for stimulation (the lower the lead impedance, the higher the impedance of the electrode/myocardium interface) and, consequently, the lifespan of the implanted device.
The impedance is a parameter that can evolve over the course of time, because it depends not only on the intrinsic characteristics of the lead (geometry, materials, etc.) but also of the electric characteristics of the electrode/myocardium interface, that also can evolve over the course of time for various reasons, in particular the evolution of the environment of the tip of the lead (e.g., the formation of reactional contact tissue such as fibrin) and the deterioration of the conducting material forming the lead electrode.
It is thus important to be able to measure the impedance of the lead regularly, to determine whether this impedance remains within acceptable limits and, if required, to readjust the electric parameters of delivery of the stimulation pulse according to the value thus measured. This measurement is made difficult by the fact that the impedance of the lead is a complex impedance, having a pure resistive component and a capacitive component, components that can vary in different ways over the course of time.
The lead impedance Zs can be modeled by a pure resistance Rs in series with a capacitance Ch called “Helmholtz capacitance.” The resistive component Rs is solely responsible for the current consumed with each stimulus, and the capacitive component Ch is, for its part, responsible for the loss by polarization to the electrode/myocardium interface.
The majority of the known implantable prostheses perform an evaluation of the complex impedance that amalgamates Rs and Ch, and that is also very imprecise and variable according to the measurement signal used: The value of the result provided by these apparatuses varies in particular according to the duration of the stimulation pulse.
The published international application WO-A-99/58192 proposes a process making it possible to determine separately the resistive and capacitance components to mitigate these disadvantages, but it requires a complex circuitry, putting in particular a shunt resistance in the stimulation circuit.